1. Field of the Invention
The present invention generally relates to magnetic disk drives, and particularly relates to a magnetic disk drive provided with a magnetic disk on which servo information is written by a stack servo track writer (SSTW) in a radial pattern in radial directions to be placed at equal intervals in the circumferential direction.
2. Description of the Related Art
An SSTW (also referred to as STW) is an apparatus designed to write servo information on a magnetic disk. Each magnetic disk on which servo information is written by an SSTW is mounted in an individual magnetic disk drive. Magnetic disks having servo information written by an SSTW in advance are mounted in magnetic disk drives, so that the productivity of the magnetic disk drives can be improved compared to the case in which servo information is written to individual magnetic disks after these disks are mounted in magnetic disk drives.
The eccentricity of a magnetic disk set in an SSTW differs from the eccentricity of the magnetic disk having servo information written thereon by the SSTW as mounted in a magnetic disk drive. Further, the tilt of the surface of a magnetic disk set in an SSTW differs from the tilt of the magnetic disk having servo information written thereon by the SSTW as mounted in a magnetic disk drive. Moreover, when a magnetic head serving as a reference in an SSTW is fixedly placed at a given position on a magnetic disk, this position does not match the corresponding position of another magnetic head on another magnetic disk. A similar phenomenon also occurs when this magnetic disk having servo information written thereon by the SSTW is mounted in a magnetic disk drive. Because of these factors, the tracks of a magnetic disk having servo information written thereon as set in an SSTW may not match and deviate from tracks (scan paths) scanned by a magnetic head on this magnetic disk having the servo information written by the SSTW as mounted in a magnetic disk drive. When a magnetic head serving as a reference in a magnetic disk drive is fixedly placed at a given position on a magnetic disk, this position does not match the corresponding position of another magnetic head on another magnetic disk, resulting in a difference in accessed track numbers. In this application, such phenomenon is referred to as an “inter-media collapse”.
Patent Document 1 discloses a method of making a track pitch vary in response to the density of the magnetic flux of the magnetic field of a voice coil motor when servo information is written to a magnetic disk. Patent Document 2 discloses a method of mounting a magnetic disk having control information written thereon by an SSTW in a magnetic disk drive. Patent Document 3 discloses a method of making a track pitch vary in response to a radial position in order to compensate for the phenomenon in which the skew angle of a magnetic head to a magnetic disk differs depending on the radial direction. Patent Document 4 discloses a method of changing a track pitch separately for each area on a magnetic disk in order to provide a sufficiently large track pitch compared to the track width in which data is written. Patent Document 5 discloses a method of writing servo information to a magnetic disk at accurate track pitches.    [Patent Document 1] Japanese Patent Application Publication No. 62-65271    [Patent Document 2] Japanese Patent Application Publication No. 3-73406    [Patent Document 3] Japanese Patent Application Publication No. 2002-237142    [Patent Document 4] Japanese Patent Application Publication No. 2003-16745    [Patent Document 5] Japanese Patent Application Publication No. 2005-100611
As the record density of a magnetic disk increases, the track pitch decreases, resulting in an adjacent track influence (hereinafter referred to as an ATI) becoming unignorable. That is, when data is written to tracks on both sides of a given track, for example, these tracks serve as a noise source with respect to the given track, which causes an error rate to increase when data is read from this given track. As the number of times data is written to the tracks on both sides increases, the deterioration of the error rate becomes increasingly conspicuous. In the worst case, data cannot be properly read from this given track.
Patent Document 3 as described above changes a track pitch separately for each area on a magnetic disk as an anti-ATI measure. Since the track pitch is constant within each area, however, it is difficult to achieve the objective of improving the record density of a magnetic disk simultaneously with the objective of suppressing an ATI.
Accordingly, there is a need for a magnetic disk drive that can improve the record density of a magnetic disk and suppress an ATI at the same time. That is, there is a need for a magnetic disk drive that can suppress the occurrence of an ATI accompanying the improvement of the record density of a magnetic disk.